This invention relates to laminated transparent assemblies, and particularly to laminated transparent assemblies provided with interlayer materials laminated to other lamina, which are composed of glass or other well-known rigid transparent materials, such as organic substitutes for glass including polycarbonates, acrylic esters, polyesters and rigid transparent polyurethanes, as is well known in the art of laminated transparencies for aircraft.
Interlayers of polyvinyl acetals such as polyvinyl butyral plasticized with well-known plasticizers, such as triethylene glycol di-2-ethyl (butyrate), dibutyl sebacate, and other monocarboxylic aliphatic acid esters of ether glycols, and interlayers composed of polyurethane resins lose their adhesive properties to glass and other rigid transparent materials that are substituted for glass in transparent laminates used as aircraft windows when the interlayers are exposed to moisture. The aircraft transparency art has sought an edge sealing material that would prevent atmospheric moisture and chemicals from penetrating into the interlayer material and cause deterioration of the bond between the interlayer and the rigid transparent layers that constitute the laminated transparency.
Various metals have been either sprayed or applied in the form of tape adhered with either a silicone adhesive or with solder. However, it is difficult to form a metal foil to conform exactly to the edge of a laminted transparency, particularly when certain layers of the transparency have their edges offset from the edges of certain other layers. When metal is sprayed onto an edge surface, it is difficult to form a continuous coating by having the metal particles coalesce. Furthermore, metal materials do not adhere well to the conventional interlayer materials. In addition, many aircraft installations comprise an electroconductive heating circuit either formed of wires carried by the interlayer or in the form of a transparent electroconductive coating of a metal or metal oxide applied to a surface of one of the rigid transparent layers that faces the interlayer. Under such circumstances, if the edge sealing composition is a metal, moisture causes a short circuit between the edge seal and the components of the heating circuit carried by the interlayer or by a transparent rigid layer. It is necessary to maintain electrical insulation between the heating circuit and the edge seal.
Non-metallic sealing compositions consisting of silicones, polysulfides, polyurethanes, acrylics, and other organic materials are not totally effective for edge sealing for aircraft, automotive and other laminated panels with interlayer materials interposed between outer lamina in laminated panels. These materials are not sufficiently impervious to moisture and chemicals that attack interlayer materials to the degree required to protect the interlayer materials from exposure to water, liquids or vapors that cause degradation of the interlayer material and can lead to edge separation, delamination, or bubbling, and can also lead to breakdown in the insulation to electrical components of a heated window and result in shorted circuits in leads, bus bars or electrical connections. Laminated assemblies using a rigid plastic substitute for at least one glass sheet also need effective sealing because of some susceptibility of rigid plastic substitutes such as polycarbonates, acrylics, and the like to moisture and chemical degradation. Moisture and chemical degradation from water, liquids or vapors can lead to crazing, craking and loss in physical and structural properties of the rigid plastic, particularly at mounting edges and at bolt holes which receive bolts that attach the transparency to an aircraft frame.
Edge barrier films are required that are thin, flexible, impervious to moisture and chemical liquids and vapor and that do not react to degrade the components of the aircraft transparency on contact therewith. Compositions having suitable vapor imperviousness include organic fluoro plastic polymers and copolymers such as polytetrafluoroethylene and fluorinated ethylene-propylene copolymers sold under the trademark TEFLON. These have the lowest moisture absorption and transmission properties (practically zero) of all organic materials tested in thin film form and also have good resistance to chemicals and excellent electrical insulating properties. However, these materials normally are not bondable, but the surfaces can be treated to make them bondable. These treated materials adhere to conventional interlayer materials such as plasticized polyvinyl acetals, particularly the butyral, and polyurethanes, but they do not adhere well to glass, acrylics, polycarbonates and other well-known rigid transparent organic plastics that are substituted for glass in certain aircraft laminated transparencies. Once the edge barrier films delaminate from the edge of the glass or rigid plastic, they expose the interlayer to atmospheric moisture and chemicals. Such exposure results in the delamination of the interlayer from the adjacent rigid transparent layers and degradation of the interlayer.